Human three-dimensional visual perception, termed stereo vision, is related to the model human observer having two eyes that are located at two slightly different positions and that form two different viewing perspectives. These two different perspectives are interpreted by the brain and, for most people, effectively “fused” to form a single perspective image. This process, often referred to as binocular fusion, operates upon the disparity between the separate images that are simultaneously formed on the two retinas. Of particular effect for binocular fusion is the relative horizontal displacement of objects in the two images. In binocular fusion, a relative depth between objects is derived, resulting in the perception of a single, broad view with depth.
Stereoscopic display systems, in existence for a number of years, are based on the observation that it is possible to simulate three-dimensional (3-D) images for perception by presenting a pair of two-dimensional images separately to each eye, each image offering a different perspective of some captured or simulated scene content. There are a number of examples of 3-D or stereoscopic displays, using various techniques for distinguishing between the image intended for the left eye and the image intended for the right eye. With any type of stereoscopic display system, some type of separation mechanism is needed in order to distinguish the left (L) and right (R) images that appear on a common display, but are respectively intended for the appropriate left and right eyes of the viewers. Left- and right-eye images can be displayed at separate times, can be of different polarizations relatively orthogonal to each other, or can be of different wavelengths. Conventional two-projector systems can use any of these separation schemes as just described. Using various techniques, single-projector digital systems can also use any of these methods.
Time-sequencing systems use a “page flipping” technique and timing for left- and right-eye image separation. Page-flipping alternately displays left- and right-eye images to provide stereo images to one or more viewers wearing shutter glasses that have left- and right-lens opacity synchronized in some manner to the display refresh rates. One example of this type of display system adapted for presenting stereoscopic images to multiple viewers is given in U.S. Pat. No. 6,535,241 (McDowall et al.).
Stereoscopic systems using polarization differences provide the left- and right-eye images using light at respectively orthogonal polarizations. Viewers are provided with polarized glasses to separate these left- and right-eye images. One example of this type of display system using linearly polarized light is given in U.S. Pat. No. 7,204,592 (O'Donnell et al.). A stereoscopic display apparatus using left- and right-circular polarization is described in U.S. Pat. No. 7,180,554 (Divelbiss et al.).
Stereoscopic systems can separate left- and right-eye images by wavelength and provide viewers with filter glasses that are suitably designed to distinguish the appropriate image for each eye. One example of this type of spectral separation display system is given in U.S. Pat. No. 7,001,021 (Jorke).
Stereoscopic systems have thus been developed to take advantage of basic principles of binocular human vision and stereoscopic display using these different approaches for distinguishing images intended for left and right eyes of a viewer and thus for simulating 3-D scene content. As experience with such systems has grown, increased attention has, correspondingly, been paid to psychophysical factors of stereoscopic perception. Considering the viewing population as a whole, it has been found that not everyone has the same perception of synthesized 3-D images. Instead, given 3-D images with left- and right-eye separation provided using any of the techniques just described, there can be considerable differences in perception and fusion of stereoscopic images from one viewer to another. For many viewers, differences in visual information or visual cues, instead of providing stereoscopic cues, can actually lead to user discomfort or to difficulty in fusing the two images, so that the left- and right-eye images are perceived by each viewer as one fused image rather than two separate images.
One difference between the visual information provided by conventional stereoscopic display systems and the real world environment arises from the fact that the viewer of a stereoscopic display must accommodate, or focus, at a single plane in space when viewing a stereoscopic display, while the disparity cues that are provided indicate that the objects are at different planes in space. This presentation differs from the real world visual environment, where the accommodative and disparity cues provide consistent information. This can be significant for stereoscopic viewing, since the vergence of our eyes and their accommodation distance are reflexively linked, often causing accommodation distance to track convergence distance as the two eyes converge to place an important object onto the fovea of each eye.
It is well known in the imaging sciences that there is an upper limit for how much left (L) and right (R) disparity the human visual system can fuse. It is also well understood that some percentage of the population is not able to interpret stereoscopic information and therefore, that people in this group are unable to benefit from the cues provided by a stereoscopic display system. It is also understood that there can be significant individual variability in an observer's ability to comfortably fuse two images that have horizontal disparity, also termed binocular disparity.
Psychophysical testing has demonstrated that the fusional range that is generally common to all users is much smaller than that for some individual users. These research findings are supported by work in optometry, where methods of quantifying the range of convergent and divergent angles that each individual is able to fuse have been applied to understand an individual's visual performance. It is well understood that this range of convergent and divergent angles that can be fused, referred to as an individual's fusional reserve, differs from one person to the next and also varies with differences in the accommodative stimulus that is provided. Importantly, research in this field has shown that some viewers can fuse a large range of convergent angles and a small range of divergent angles while, in contrast, other individuals can fuse larger ranges of divergent angles than convergent angles. Because of this individual variability, if a stereoscopic system is developed to provide comfortable stereoscopic images to all viewers, the range of disparity that can be shown should be limited to a range that includes a large percentage of the viewing population, but, in doing so, will tend to exclude at least some portion of potential viewers.
Addressing this difficulty, commonly-assigned U.S. Patent Application Publication No. 2005/0190180 (Jin et al.) describes flexible rendering of stereoscopic images that is conditioned according to the stereoscopic fusing capability of the observer. This approach addresses the problem of accommodating the ability of a viewer to fuse stereo images by customizing the presentation of an image for a single viewer looking at a single display.
It has been recognized that there can be stereoscopic display applications for which it is useful to alter the perspective image content that is provided to each of a number of multiple viewers. For example, U.S. Patent Application Publication No. 2008/0036854 (Elliot et al.) describes a method of communicating and rendering either stereoscopic images or dual-view images, in which two viewers are enabled to see different images on the same display through temporal multiplexing and proper synchronization between image projection apparatus and electronic shutter glasses. The display apparatus described in the Elliot et al. '6854 disclosure, however, is limited to two viewers only, presents either stereoscopic images or dual-view images for simultaneous viewers, and makes no provision for psychophysical differences or preferences between viewers.
As a general rule, stereoscopic displays, because they must share light between separate right- and left-eye images, suffer from a lack of brightness. This deficiency applies whether these images are differentiated by multiplexed timing, polarization separation, or spectral range separation.
Conventional solutions for stereoscopic viewing by two or more viewers are limited to simultaneously showing either stereoscopic or dual-view images within a viewing session and fail to address the need for adaptation to individuals in its audience. The compromises that have been made in order to provide 3-D viewing with existing solutions, targeting only a portion of the viewer population, mean that a percentage of the audience may be left without the advantages of stereoscopic viewing or may find it visually uncomfortable. Further, conventional solutions are unable to provide stereoscopic images having satisfactory brightness levels to two or more subsets of viewers.